In recent years, silicon photonics has attracted attention as a promising technology for large capacity interconnection and it is expected to increase the transmission capacity per optical wire within Si chips through wavelength division multiplexing (WDM).
In order to transmit and receive WDM optical signals within a Si chip, it is necessary to multiplex (MUX) or demultiplex (DeMUX) the WDM optical signals by using a wavelength multiplexer/demultiplexer (MUX/DeMUX) if necessary. Usually, Si wire waveguides have a very large structural birefringence. Therefore, the transmission properties of a MUX/DeMUX formed of Si wire waveguides differ significantly depending on the polarization state of optical signals. That is to say, normal operation is possible only in the polarization state of either the TE mode or the TM mode.
Meanwhile, the polarization state is not kept constant in the transmission path of optical signals. Therefore, optical signals that enter into a light-receiving unit formed of a Si wire DeMUX and a light receiver (photodiode) are polarized into random components and thus deterioration of the reception properties is inevitable depending on the polarization state.
In order to overcome this problem, a WDM polarization diversity configuration that includes Si wire waveguides has been proposed. Here, a conventional wavelength division multiplexing optical receiver is described in reference to FIG. 12. FIG. 12 is a schematic plan diagram illustrating a conventional wavelength division multiplexing optical receiver, where a WDM optical signal that has entered into an input waveguide 71 made of a silicon wire waveguide is divided into TE beam and TM beam of which the polarization planes are orthogonal to each other by means of a directional coupler type polarization beam splitter (PBS) 72 made of silicon wire waveguides so as to be outputted into a loop waveguide 73 made of a silicon wire waveguide. The TM beam has its polarization plane rotated 90° by an eccentric double core type polarization rotator (PR) 74 made of a silicon wire waveguide inserted into the loop waveguide 73 and is outputted as TE* beam. In contrast, the TE beam, which has been divided by the polarization beam splitter 72, retains its polarization plane as it is guided through the loop waveguide 73.
TE beams that are guided in opposite directions to each other through the loop waveguide 73 are demultiplexed to the respective wavelengths due to the demultiplexing functions of add-drop micro-ring resonators (AD-MRRs) 751 and 752 wherein the optical paths thereof differ in length. The respective TE beams that have been demultiplexed are outputted into loop waveguides 761 and 762 having polarization rotators 771 and 772 and optical path length compensation waveguides 781 and 782.
From among the outputted TE beams, the TE* beam that is guided through the loop waveguide 761 or 762 in the clockwise direction in the figure has its polarization plane rotated 90° by the polarization rotator 771 or 772 so as to be outputted as a TM beam and is inputted into a polarization multiplexer 791 or 792. At this time, the TE beam that is guided through the loop waveguide 761 or 762 in the counter-clockwise direction in the figure is inputted into the directional coupler type polarization multiplexer 791 or 792 at a timing that is made to match the TM beam by the optical path length compensation waveguide 781 or 782 so as to be multiplexed (MUX), and is divided into the respective wavelengths when outputted from the output waveguide 801 or 802 so as to be received by light receivers (not shown).
In this case, the MDW beams that enter the AD-MRR have a constant polarization state, that is to say in the TE mode or in the TE* mode, and therefore the deterioration in the properties due to the demultiplexing (DeMUX) can be prevented. Accordingly, WDM optical signals can be multiplexed or demultiplexed without being affected by the polarization state of the WDM optical signals that enter the AD-MRR.